1. Field of the Invention
The present invention relates in general to filling or plugging of drill holes. In particular, the present invention relates to an improved method for plugging an abandoned drill hole within the earth and for maintaining the plug integrity indefinitely.
2. Description of the Related Art
It has been well known to provide deep (on the order of several hundred feet) drill holes within the earth for a variety of purposes. Such holes are typically formed during a standard oil well drilling operation. The drill hole is formed and then lined with a casing. The drill hole passes through several compositions, such as hard compacted soil, clay, loose sand, and other typical geologic material, in addition to one or more water bearing layers. Such water bearing layers may represent a saline water source or a fresh water aquifer. Once an oil reserve is exhausted, the oil well hole is abandoned. If left unattended, gases and fumes escape through the hole into the atmosphere. Further, the casings erode and crack, thereby causing damage to the aquifier levels and the like.
In particular, a fresh water aquifer may "leak" downward through the casing and hole into a fracture or uncharged zone, causing loss of water from the aquifer. A drill hole extending between a saline water source and a fresh water aquifer may allow commingling of these water supplies, damaging both. Additionally, contamination from the surface may cause damage, such as surface rain water passing downward through the hole and casing into a fresh water aquifer.
To overcome these problems it has been known to plug the casings and drill holes with concrete. However, concrete has proven less than effective in maintaining the integrity of the seal throughout the casing over long periods of time. First as the concrete dries, it contracts thereby separating (pulling away) from the casing. Second, as the concrete ages it cracks. These age cracks and contractions during curing produce voids within and about the concrete which allow gases to escape and allow contamination of the water level. Hence, concrete plugs have met with limited success.
In the past, finely ground chemically unaltered sodium bentonite, has been proposed for filling shallow holes of up to 30 meters or 100 feet deep. Heretofore, a system or method has not been proposed which effectively utilizes sodium bentonite to fill holes several hundred feet deep, such as oil well holes. A report entitled "Axial Shear Strength Testing of Bentonite Water Well Annulus Seals" by Fred Lee Ogden and James F. Ruff published by Colorado State University, 1989, discusses the use of bentonite as an annulus sealant. Past usage of bentonite is explained in a report entitled "Experiments in Subsurface Applications of Bentonite in Montana" by John Wheaton, Steve Regele, Bob Bohman, Dave Clark and Jon Reiten, published by Montana Bureau of Mines and Geology, 1994. Both of the foregoing reports are incorporated herein by reference.
A first and simple method for placing the bentonite in a shallow hole without a casing is to simply pour a small granular form of dry bentonite (i.e., less than 3/8" in diameter) into the drill hole from the surface. The bentonite falls downward through the drill hole, filling the hole from the bottom upward. However, where the drill hole passes through unconsolidated material, such material may form a cave in at the sides of the drill hole, forming a plug at a position spaced above the bottom of the hole. In such cases the small granular bentonite will simply fill the hole from the plug upward and not pass downward to the bottom of the hole to fully plug the hole. Additionally, it is not possible to pour bentonite in the foregoing manner into drill holes passing through high volume artisan flows, or in drill holes using a dug pit (i.e. where a bentonite slurry has been employed to maintain wall integrity in the hole).
When the small or finely ground bentonite of 3/8" in diameter is poured into the hole, it begins to expand when exposed to water. The conventional finely ground pouring method is adequate for shallow holes since the bentonite sinks to the bottom of the hole before a significant amount of swelling occurs. However, if bentonite is poured into deep oil well holes, the hole may contain several hundred feet of water.
In general, high-grade and low-grade bentonite chips fall at an average velocity of 1.0 ft/sec. Smaller bentonite granules of 3/8" in diameter or less have more surface area per unit weight, and therefore fall at a slower rate. This is due to the fact that smaller bentonite granules are typically less dense than larger chips. For instance, a bentonite granule with a diameter of 3/8" may have a volume of 0.5 cm.sup.3 and weigh 1 chip with a 3/4" diameter weights 3.65 grams and has a volume of 1.50 cm.sup.3. In this example, the smaller granule has a density of 2.02 gr/cm.sup.3 and the larger chip has a density of 2.43 gr/cm.sup.3.
Further, once hydration begins, the density of the granule decreases as the granule swells. Similarly, the fall velocity of the granule in water decreases at a rate of about 0.009 ft/sec per minute of fall. For instance, a granule having an initial fall velocity of just under 1 ft/sec, after 44 minutes of exposure to water, with fall at approximately 0.6 ft/sec. As the granule absorbs water, its density decreases approaching the density of water further slowing the fall velocity. These factors prevent small granules from effectively being used to plug deep holes with several hundred feet of water therein.
The conventional form of bentonite poured into shallow holes is formed of small granular particles having a diameter of no greater than 3/8 inches. Such small material has proven ineffective when poured into holes having high fluid flow rates therethrough and when poured into deep holes retaining a high liquid level (i.e., a long distance between the hole bottom and liquid level). As the small granular material passes through the liquid, it begins to hydrate and swell. Granular bentonite having a diameter of no greater than 3/8 inches swells quickly and reaching its liquid limit (i.e., the saturation point of the bentonite). If the bentonite swells beyond its liquid limit, the bentonite turns to a slurry state. At the liquid limits, the bentonite lacks sufficient additional swelling ability to achieve seal within the hole. Hence, conventional small granular material is ineffective for filling deep holes. Additionally, the conventionally sized granular bentonite falls through the liquid in the hole in an unconcentrated state. Each granular particle is afforded a large portion of the cross-sectional area of the hole within which to expand. Sodium bentonite will continuously expand until it is restrained by its surrounding environment or starved for water. Once the bentonite expands to a size several times its dehydrated size, the conventionally sized bentonite granule loses its solid structure and turns to a slurry liquid state. Once sodium bentonite hydrates to the point that it turns to a slurry liquid, the granule becomes ineffective at plugging holes.
Past systems that use the conventional sized bentonite particles have prevented degradation to this slurry state by filling the hole with dehydrated granular particles before each individual particle is allowed to expand substantially. To do so, the granules are poured into shallow holes or holes having very little liquid standing therein. In shallow holes, conventionally sized particles collect in the bottom of the hole before expanding substantially. However, when conventionally sized granular bentonite is poured into deep holes and through deep liquid levels, each individual particle turns to a slurry state before reaching the bottom of the hole and collecting with the other falling particles.
A second and more reliable method is to insert a conduit into the drill hole and pass a slurry of bentonite through the conduit while slowly withdrawing the conduit. For example, U.S. Pat. No. 5,013,191 to Kitanaka discloses a special auger which is rotated in the normal manner to drill the hole, and then is fixed against rotation while the bentonite slurry is passed through a central hole in the auger and the auger is withdrawn. While this method is effective, it requires the use of a special and expensive auger and is limited to shallow applications.
An alternative slurry/conduit method consists of simply inserting a standard 11/2 inch PVC pipe into the drill hole and passing the slurry through this pipe. While this method does not require the use of a special auger, if the hole has been plugged as noted above, the method requires an initial step of drilling with an auger to clear the plug prior to inserting the pipe. Again, this system is limited to shallow applications.
Moreover, problems have been encountered with the above systems when using a mixture of heavy bentonite gel water slurry. The slurry mixture is used while drilling the holes to keep the walls of the drill holes from sluffing inward, thereby avoiding the need to reconstruct sluffed areas within the hole. After abandonment, the slurry stands within the hole. The density of the slurry is sufficiently close to the density of conventional granular bentonite which is poured directly into the hole, that the slurry holds the granular bentonite in suspension proximate the top section of the hole. Thus, when the granular bentonite is poured into the hole, it does not sink to the bottom, and thus does not plug the hole from the bottom up.
Moreover, the foregoing systems are ineffective when used with wet auger drilled holes which utilize water injected from the surface downward into the hole. While drilling the hole, the agitation of the auger stem, when combined with the injected water, creates a heavy native mud material that remains within the hole after drilling is completed. The density of this mud is relatively high, with respect to that of bentonite granular material, and thus holds the bentonite granular material in suspension at the top of the hole.
Finally, in any plugging application (shallow or deep) utilizing bentonite, the bentonite retains the ability indefinitely to expand or contract depending upon its degree of hydration. Thus, the bentonite remains in a swollen state to plug the hole so long as it remains adequately hydrated. If the bentonite dries out, it contracts, thereby compromising the seal within the hole. Therefore, it is necessary to ensure that the bentonite is continuously and indefinitely exposed to liquid or at least used in an application designed to prevent liquid loss. Past shallow applications using fine bentonite have failed to acknowledge or address this problem, in part due to the fact that shallow holes are generally exposed to a natural water table which provides a continuous source of liquid. Further, past shallow fine bentonite applications utilize a column of bentonite less than 100 feet long within a hole which does not necessarily contain a casing. In these shallow applications, natural water from the water table and run-off water from the surface provide adequate and indefinite hydration of the entire bentonite column.
However, in deep holes much longer columns of bentonite are required. Plus, when a hole casing is used, so long as it retains its integrity, the interior of the column is isolated from ground water and underground water tables. Hence, any bentonite within the column may dry out and contract, thereby compromising the seal integrity.
A need remains within the industry for an improved method and apparatus for plugging abandoned drilled holes. It is an object of the present invention to meet this need.